I am still playing Okami, but you may have it on another system. The lego games fall into this category.
Smash Bros is good as long as you have friends.
Super Mario Galaxy is pretty good as mario games got, same goes for Zelda: TP. I should hope this is also true of Metroid 3, but I have not played it.
I have heard good things about DeBlob, Madworld, and No More Heros. I have not played these.
The Conduit is supposed to be good on the Wii, but average when compared to what other consoles are providing. They are making another game, I forget what it's called.
Super Punch-Out has been receiving loads of praise, but this may be colored by nostalgia. Nintendo remakes in general may fall into this category.
Rockband is available, but far superior on the other systems.
Metroid Prime 3 is god amongst Wii games. You should probably rent The Conduit, Mario Galaxy is amazing. Zelda blah blah blah. Oh yeah and Punch-Out!!. That game is great.
Yeah all the ones mentioned here are fantastic. I'll just add that I've had more fun playing Mario Galaxy than any other game before. It's not the best game ever, but it is the most fun in my opinion. Mario Kart is always pretty decent but get a Gamecube controller or two and the Gamecube version if you can, as I think that that's superior. They should both be pretty damn cheap, though perhaps difficult to find.
Everything above mentioned is good. I highly recommend Okami and most everything else here. If your looking for a party game try Boom Blox it's simple fun. Also go online and look for oldschool games to download. Me and some of my friends have lots of fun playing Bomberman.